Sooner or later, you'll want to reset your log files (access_log and error_log) because they are too big, or full of old information you don't need.
Most people's first attempt at replacingthe logfile is to just move the logfile or remove the logfile. This doesn't work.
Apache will continue writing to the logfile at the same offset as before the logifile moved. This results in a new logfile being created which is just as big as the old one, but it now contains thousands (or millions) of null characters.
The correct procedure is to move the logfile, then signal Apache to tell it to reopen the logfiles.
Apache is signalled using the SIGHUP (-1) signal.
e.g.
mv access_log access_log.old ; kill -1 `cat httpd.pid`
n.b. httpd.pid is a file containing the process id of the Apache httpd daemon, Apache saves this in the same directory as the log files.
Many people use this method to replace (and backup) their logfiles on a nightly basis.